In a next generation mobile communication system, a system capacity as much as hundreds of Mbps is necessary. To realize this system capacity, a large number of groups of small cell base stations which cover areas of dozens of meters in radius have to be laid. The cost for such a large number of base stations is hard to afford. Wireless backhaul radically reduces the cost for laying cables necessary for installing the system by forwarding packets between base stations via multihop network. One could say that wireless backhaul is the only solution to the cost problem.
A wireless backhaul includes some core nodes and pluralities of slave nodes. Only core nodes are connected with backbone network via cables. Each slave node is connected to the backbone network by wireless multihop forwarding to a core node. Such a constitution realizes laying base stations at low cost and easy rearrangement or addition of base stations.
The performance of a wireless backhaul is determined by the efficiency of forwarding packets between nodes. The present inventors have proposed Intermittent Periodic Transmit (IPT) for increasing the efficiency of forwarding packets in a wireless backhaul (See Patent Literature 1, Non Patent Literature 1 and so on). In a wireless backhaul, the network forwarding performance deteriorates mainly by packet collision caused by two nodes interfering each other when they transmit packets at the same time. In IPT, a source node transmits packets to a destination node at a particular period (IPT transmission period). When each node on a routing path receives a packet, it forwards the packet immediately to the next node. By making a source node transmit packets at a particular period, it is possible to control a frequency reuse distance, which is a distance of two nodes which can transmit packets at the same time without packet collision (See FIG. 6).
A frequency reuse distance is proportional to IPT transmission period. If an IPT transmission period is more than a particular value, the interference between transmission source nodes can be avoided. When IPT transmission period is set to just a threshold value, the throughput from the source node to the destination node becomes maximum. Thus, IPT transmission period is the most important parameter to adopt IPT in a wireless backhaul. Inadequate value of IPT transmission period cannot realize the optimal performance of the system. Here, IPT transmission period is influenced by surrounding environment, the installation site or antenna direction of each node, and so on. Therefore, it is difficult to set an adequate value of IPT transmission period. The method to determine IPT transmission period automatically is desired.
In Patent Literature 1 and Non Patent Literature 1, the protocols to calculate IPT transmission period automatically for a wireless backhaul are proposed. More specifically, three kinds of packets (RTSS (Request To Stop Sending) packet, CTP (Clear To Pilling up) packet, and CTPACK (CTP ACKnowledgement) packet) are defined. Referring to FIG. 7, conventional period determination process is described. (1) A source node transmits packets at a particular transmission interval to a destination node. (2) During forwarding packets, the flow of packets can be interrupted at a forwarding node by interference or the like. The node where interruption has occurred (a congested node in FIG. 7) immediately stops forwarding packets and transmits an RTSS packet to the source node. (3) When a source node receives the CTP packet, the source node transmits a CTP packet. When each forwarding node receives the CTP packet, each of the forwarding nodes clears up packets. Then, all the packets in the routing path is cleared up. (4) When a destination node receives the CTP packet, the destination node transmits a CTPACK packet to the source node. (5) The source node stops transmitting packets during the period of transmitting the CTP packet to receiving the CTPACK packet. When the source node receives the CTPACK packet, the source node restarts to transmit packets (packets DAT1, DAT2 . . . ) at a little more transmission interval. By repeating the process from (1) to (5) until packet collision does not occur, the transmission interval between each slave node converges to a certain value. The converged value is determined as the IPT transmission period of the destination node.